Implantable medical devices (IMDs) for monitoring a physiological condition or delivering a therapy typically rely on one or more sensors positioned in a patient's blood vessel, heart chamber, or other portion of the body. Examples of IMDS include heart monitors, pacemakers, implantable cardioverter-defibrillators (ICDs), myostimulators, nerve stimulators, drug delivery devices, and other IMDs where such sensors are desirable. Implantable sensors used in conjunction with an IMD generally provide a signal related to a physiological condition from which a patient condition or the need for a therapy can be assessed.
Measurement of blood oxygen saturation levels are of interest in determining the metabolic state of the patient. Generally, a decrease in blood oxygen saturation is associated with an increase in physical activity or may reflect insufficient cardiac output or respiratory activity. Thus monitoring blood oxygen saturation allows an implantable medical device to respond to a decrease in oxygen saturation, for example by pacing the heart at a higher rate. An implantable oxygen sensor for use with an implantable medical device is generally disclosed in commonly assigned U.S. Pat. No. 6,198,952 issued to Miesel, hereby incorporated herein by reference in its entirety. Cardiac pacemakers that respond to changes in blood oxygen saturation as measured by an optical sensor are generally disclosed in U.S. Pat. No. 4,202,339 issued to Wirtzfeld and in U.S. Pat. No. 4,467,807 issued to Bornzin.
One limitation encountered with the use of implantable optical sensors can arise as the result of tissue encapsulation of the sensor that occurs as a result of the body's normal response to a foreign object. If an optical blood oxygen sensor is positioned in an area of relatively high blood flow, tissue encapsulation of the sensor may not occur or may at least be minimized to a thin collagenous sheath. If a blood oxygen sensor resides in an area of relatively low blood flow or a stagnant area, tissue encapsulation is likely to occur and the capsule may become relatively thick. Such tissue overgrowth interferes with the performance of the sensor in accurately measuring blood oxygen or other metabolites by reducing the (light) signal to noise ratio. For example, the light signal associated with blood oxygen saturation is reduced due to attenuation of emitted light from the optical that reaches the blood volume and attenuation of the reflected light from the blood volume reaching a light detector included in the optical sensor. Noise due to extraneous light reaching the light detector is increased by the scattering of emitted light by the tissue overgrowth.
The time course and degree of tissue encapsulation of an optical sensor, or any other medical device implanted within the blood volume, is uncertain. Thrombus formation in the vicinity of the sensor due to blood stasis or endothelial injury can occur at unpredictable times after device implant. If the thrombus is in contact with the endocardium or endothelium, macrophages can invade the clot, phagocytose the blood cells and orchestrate collagenous encapsulation by fibroblasts. Because the time course and occurrence of these events is unpredictable, the reliability of blood oxygen saturation measurements at any point in time may be uncertain.
One approach to solving the problem of tissue overgrowth is generally disclosed in U.S. Pat. No. 6,125,290 issued to Miesel, incorporated herein by reference in its entirety. A self-test light detector is provided for estimating the amount of light reflected back into a light emitter portion instead of being transmitted through a lens for reflection from a blood volume. An output signal from self-test light detector may be employed to calibrate or adjust the output signal provided by a light detector in a manner that the estimate of blood oxygen saturation is compensated or adjusted to account for the degree or amount of tissue overgrowth of the sensor.
A need remains, however, for a method for adjusting a blood oxygen saturation (or other metabolite) measurement to account for extra light intensity associated with light scattering tissue or thrombus over the oxygen sensor, and the like. The method preferably provides accurate blood oxygen saturation measurement independent of the presence of tissue overgrowth.